This invention relates to battery charging circuits. More specifically, it relates to charging circuits for use with small rechargeable batteries of the type utilized in power tools. Typically, these cells are nickel-cadmium sealed cells and may be charged hundreds of times before replacement is necessary. A further attribute of such cells is that they are capable of accepting very high charge currents when discharged. As the cells reach full charge the charge rate must be reduced to prevent excessive gas evolution which could cause the cells to rupture. In particular the cell parameters of temperature and voltage change dramatically during fast charging and must be monitored in order to insure that the cell is undamaged by the charging process.
In the past, circuits have been proposed which monitor cell temperature and, upon the reaching of a preselected value, discontinue fast charge operation. Similarly, circuits have also been proposed which monitor the voltage of the cell during the fast charge. None of the prior techniques have been entirely satisfactory and safe when it is desired to charge the cells at a high rate such as the 4C charge rate. Generally, the 4C rate may be defined as four times a rate which would charge the cell in one hour.
Where is is desired to utilize 4C charge rates, it is necessary to provide both voltage and temperature cut off to insure safe charging operation over a range of temperature and voltage conditions which may be encountered when such units are recharged by a consumer. At low temperatures voltage is usually the controlling factor for detecting when the fast charge rate should be terminated to prevent cell damaging overcharge. At high temperatures the battery temperature is the controlling factor with cell temperature rising beyond a safe value before overcharging takes place. By providing a circuit with both voltage and temperature cut off it is possible to charge cells at the 4C rate while assuring maximum cell life and consumer safety. For even further protection it is desirable to limit the charge period to a selected value.
It is known to employ fast charge charging circuits with both voltage and temperature cut offs. However, such circuits require that the charging current be periodically interrupted to permit measurement of the trough voltage of the batteries under charge. This interruption is undesirable in a circuit whose primary purpose is to quickly charge the battery pack so that it may be reused in a portable tool as, for example, a drill, saw, etc.
It is accordingly an object of the present invention to provide a fast charging circuit for batteries having at least temperature and voltage cut off and which maintains the 4C charging rate until the occurrence of one of the trigger conditions.
It is a further object of the invention to provide a fast charge circuit which is simple in operation, highly reliable and safe for use by purchasers of small power tools.
Another object of the invention is to provide a battery charging circuit which monitors the trough voltage of the cells without interrupting the charging cycle and terminates charging when the trough voltage exceeds a selected reference voltage.
It is a further object of the invention to provide a fast charge circuit which employs voltage comparators to monitor the trough voltage and the charging voltage whereby the value of the trough voltage can be monitored to determine the state of charge of the cells during appropriate "windows" on the charging voltage waveform.
These and other objects and advantages of the invention will be apparent from the remaining portion of the specification.